Active Model Basics =================== This guide should provide you with all you need to get started using model classes. Active Model allows for Action Pack helpers to interact with non-Active Record models. Active Model also helps building custom ORMs for use outside of the Rails framework. After reading this guide, you will know: -------------------------------------------------------------------------------- Introduction ------------ Active Model is a library containing various modules used in developing frameworks that need to interact with the Rails Action Pack library. Active Model provides a known set of interfaces for usage in classes. Some of modules are explained below. ### AttributeMethods The AttributeMethods module can add custom prefixes and suffixes on methods of a class. It is used by defining the prefixes and suffixes and which methods on the object will use them. ```ruby class Person include ActiveModel::AttributeMethods attribute_method_prefix 'reset_' attribute_method_suffix '_highest?' define_attribute_methods 'age' attr_accessor :age private def reset_attribute(attribute) send("#{attribute}=", 0) end def attribute_highest?(attribute) send(attribute) > 100 end end person = Person.new person.age = 110 person.age_highest? # true person.reset_age # 0 person.age_highest? # false ``` ### Callbacks Callbacks gives Active Record style callbacks. This provides an ability to define callbacks which run at appropriate times. After defining callbacks, you can wrap them with before, after and around custom methods. ```ruby class Person extend ActiveModel::Callbacks define_model_callbacks :update before_update :reset_me def update run_callbacks(:update) do # This method is called when update is called on an object. end end def reset_me # This method is called when update is called on an object as a before_update callback is defined. end end ``` ### Conversion If a class defines `persisted?` and `id` methods, then you can include the `Conversion` module in that class and call the Rails conversion methods on objects of that class. ```ruby class Person include ActiveModel::Conversion def persisted? false end def id nil end end person = Person.new person.to_model == person # => true person.to_key # => nil person.to_param # => nil ``` ### Dirty An object becomes dirty when it has gone through one or more changes to its attributes and has not been saved. This gives the ability to check whether an object has been changed or not. It also has attribute based accessor methods. Let's consider a Person class with attributes `first_name` and `last_name`: ```ruby require 'active_model' class Person include ActiveModel::Dirty define_attribute_methods :first_name, :last_name def first_name @first_name end def first_name=(value) first_name_will_change! @first_name = value end def last_name @last_name end def last_name=(value) last_name_will_change! @last_name = value end def save # do save work... changes_applied end end ``` #### Querying object directly for its list of all changed attributes. ```ruby person = Person.new person.changed? # => false person.first_name = "First Name" person.first_name # => "First Name" # returns if any attribute has changed. person.changed? # => true # returns a list of attributes that have changed before saving. person.changed # => ["first_name"] # returns a hash of the attributes that have changed with their original values. person.changed_attributes # => {"first_name"=>nil} # returns a hash of changes, with the attribute names as the keys, and the values will be an array of the old and new value for that field. person.changes # => {"first_name"=>[nil, "First Name"]} ``` #### Attribute based accessor methods Track whether the particular attribute has been changed or not. ```ruby # attr_name_changed? person.first_name # => "First Name" person.first_name_changed? # => true ``` Track what was the previous value of the attribute. ```ruby # attr_name_was accessor person.first_name_was # => "First Name" ``` Track both previous and current value of the changed attribute. Returns an array if changed, else returns nil. ```ruby # attr_name_change person.first_name_change # => [nil, "First Name"] person.last_name_change # => nil ``` ### Validations Validations module adds the ability to class objects to validate them in Active Record style. ```ruby class Person include ActiveModel::Validations attr_accessor :name, :email, :token validates :name, presence: true validates_format_of :email, with: /\A([^\s]+)((?:[-a-z0-9]\.)[a-z]{2,})\z/i validates! :token, presence: true end person = Person.new(token: "2b1f325") person.valid? # => false person.name = 'vishnu' person.email = 'me' person.valid? # => false person.email = 'me@vishnuatrai.com' person.valid? # => true person.token = nil person.valid? # => raises ActiveModel::StrictValidationFailed ``` ### ActiveModel::Naming Naming adds a number of class methods which make the naming and routing easier to manage. The module defines the `model_name` class method which will define a number of accessors using some `ActiveSupport::Inflector` methods. ```ruby class Person extend ActiveModel::Naming end Person.model_name.name # => "Person" Person.model_name.singular # => "person" Person.model_name.plural # => "people" Person.model_name.element # => "person" Person.model_name.human # => "Person" Person.model_name.collection # => "people" Person.model_name.param_key # => "person" Person.model_name.i18n_key # => :person Person.model_name.route_key # => "people" Person.model_name.singular_route_key # => "person" ```